Rectifier



y 1949. w. E. BLACKBURN 2,476,300

RECTIFIER Filed May 7, 1946 Selenium WITNESSES: INVENTOR Patented July 19, 1949 RECTIFIER Wayne E. Blackburn, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 7, 1946, Serial No. 667,986

4 Claims.

My invention relates to selenium rectifiers and, in particular, relates to a method of producing a blocking layer at the rectifying surface between the selenium and the counter-electrode. In the selenium rectifier art, various methods have been tried for producing a barrier layer at the selenium surface which would produce a higher ratio of conductivity of the rectifier in the conductive direction to conductivity in the non-conductive direction. It is, of course, a pre-requisite quality of such layers that they should be stable, and should undergo only a slow change with time if any aging at all takes place. For example, the surface of the selenium adjacent the counterelectrode, i. e., the interface at which rectification takes place, has been coated with various kinds of organic material usually applied from a liquid solution. Difiiculty has been found, however, with such barrier layers because of the alkaline or reducing action characteristic of the material. It was found to be quite characteristic of such layers that they aged rapidly because of the very activity with which they reacted on the selenium when first applied thereto. In short, selenium rectifiers made by such a process were found to change in electrical characteristics with time at an undesirably rapid rate.

One object of my present invention is, accordingly, to produce a barrier layer by the use of an organic material which is incapable of producing an alkaline or reducing action.

Another object of my invention is to produce a barrier layer for selenium rectifiers which has a high degree of stability during the active life of the rectifier. I

Still another object of my invention is to provide a method of making a barrier layer for selenium rectifiers which results in giving these rectifiers a longer satisfactory life than was characteristic of rectifiers of the prior art.

Other objects of my invention will become apparent upon reading the following description when taken in connection with the drawing, in which the single figure shows a cross-section of a selenium rectifier embodying the principles of my invention.

Referring in detail to the drawing, the preferred method of employing my invention is to nickelplate and sand-blast a base plate I of steel, or other suitable material. The plate is then coated with a layer 2 of amorphous selenium by dipping the plate in a bath of the molten selenium having a. temperature of about 350 C., and thereafter rotating the disc about an axis perpendicular thereto, to remove excess selenium by the action of (Cl. 1753B6) centrifugal force. This leaves a thin, substantially uniform layer of the selenium covering the steel disc I. For most purposes, it is desirable to coat only one side of the steel plate with selenium, and this may conveniently be accomplished by mounting two steel discs, back to back, before dipping them into the molten selenium and subsequently centrifuging them. The coated discs are then submerged in a hot solution of an organic compound which has no alkaline or reducing action on selenium. As an example of such compounds, I will give polyvinyl alcohol, polyvinyl acetate and cellulose acetate. The temperatures of the solutions should be, for these three materials, respectively, C., 60 C., and 60 C. It will be found that immersion in such solution for a period of around 30 seconds will result in production of a continuous film 3 on the surface of the selenium which will act as an effective barrier layer in accordance with my invention. After removal from the solution, and washing with distilled water and drying, a counter-electrode 4 comprising preferably an alloy comprising approximately of cadmium and 20 tin, is deposited on the surface of this film, for example, by Schoop-spraying. The unit is then subjected to a heat-treatment, preferably at about 210 C. for about 2 hours, to convert the amorphous selenium into the crystalline form. Such heat-treatment likewise bakes the organic film between the counter-electrode and the selenium, thereby forming a stable barrier layer.

The rectifier is then preferably subjected to electrical forming by impressing thereon an alternating voltageof 36 volts at sixty cycles per second for around 60 minutes.

I claim as my invention:

1. The method of forming a barrier layer on the selenium surface for a selenium rectifier which comprises the steps of immersing a backing plate coated on a portion of its surface with amorphous selenium in a hot solution of a compound selected from the group comprising polyvinyl alcohol, polyvinyl acetate and cellulose acetate, thereafter applying a counter-electrode to the surface of the so-treated selenium and thereafter heating the unit at a temperature in the neighborhood of 210 C., sufliciently to convert said selenium to the crystalline form.

2. The method of forming a barrier layer on the selenium surface for a selenium rectifier which comprises the steps of immersing a backing plate coated on a portion of its surface with amorphous selenium in a hot solution of a compound selected from the group comprising polyvinyl alcohol,

polyvinyl acetate and cellulose acetate, the temperature of said solution being in the neighborhood of 60 (2., thereafter applying a counter-electrode to the surface of the so-treated selenium and thereafter heating the: unit at a temperature in the neighborhood of 210 C., sufliciently to convert said selenium to the crystalline form.

3. The method of forming a barrier layer on the selenium surface for a selenium rectifier which comprises the steps of immersing a backing plate coated on a portion of its surface with amorphous selenium in a hot solution oi a compound selected from the group comprising polyvinyl alcohol, polyvinyl acetate and cellulose acetate for a period in the neighborhood of 80 seconds, the temperature of said solution being in the neighborhood of 60 C., thereafter applying a. counter-electrode to the surface of the so-treated selenium and thereafter heating the unit at a temperature in the neighborhood of 210 C. for a period in the neighborhood of 120 minutes.

4. A selenium rectifier having sandwiched between the selenium and the counter-electrode a. barrier layer comprising the reaction product of amorphous selenium with a hot solution of one of the following group of materials: polyvinyl alcohol, polyvinyl acetate and cellulose acetate, said barrier layer being baked subsequent to the application of the counter-electrode at a temperature in the neighborhood of 210 C., sufliciently to 10 convert said selenium to the crystalline form.

WAYNE E. BLACKBURN.

REFERENCES CITED The following references are of record in the 15 file of this patent:

UNITED STATES PATENTS Number Name Date 2,124,306 Osawa July 19, 1938 20 2,139,731 De Boer Dec. 31, 1938 2,426,242 Saslaw Aug. 26, 1947 

